Adaptive frequency touchscreen controller using intermediate-frequency signal processing

ABSTRACT

A method and apparatus for adapting an acoustic touchscreen controller to the operating frequency requirements of a specific touchscreen are provided. The adaptive controller can either utilize look-up tables to achieve the desired output frequency or the it can use a multi-step process in which it first determines the frequency requirements of the touchscreen, and then adjusts the burst frequency characteristics, the receiver circuit center frequency, or both in accordance with the touchscreen requirements. In one embodiment, the adaptive controller compensates for global frequency mismatch errors. In this embodiment a digital multiplier is used to modify the output of a crystal reference oscillator. The reference oscillator output is used to control the frequency of the signal from the receiving transducers and/or to generate the desired frequency of the tone burst sent to the transmitting transducers. In another embodiment that is intended to compensate for both global and local frequency variations, the adaptive controller uses a digital signal processor. The digital signal processor, based on correction values contained in memory, defines a specific center frequency which preferably varies according to the signal delay, thus taking into account variations caused by localized variations in the acoustic wave reflective array. In yet another embodiment, a non-crystal local oscillator is used to provide the reference signal in the adaptive controller. The use of such an oscillator allows the controller to be miniaturized to a sufficient extent that it can be mounted directly to a touchscreen substrate. A feedback loop is used to compensate for oscillator drift. A discriminator circuit determines the degree of deviation from the desired frequency. The output from the discriminator is used to adjust the frequency of the local oscillator such that it tracks the frequency of the touchscreen.

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to touchscreens and, moreparticularly, to a method and apparatus for adapting the frequency of atouchscreen controller in order to match the controller to theparticular operating characteristics of a specific touchscreen.

BACKGROUND OF THE INVENTION

Touchscreens are used in conjunction with a variety of display types,including cathode ray tubes (i.e., CRTs) and liquid crystal displayscreens (i.e., LCD screens), as a means of inputting information into adata processing system. When placed over a display or integrated into adisplay, the touchscreen allows a user to select a displayed icon orelement by touching the screen in a location corresponding to thedesired icon or element. Touchscreens have become common place in avariety of different applications including, for example, point-of-salesystems, information kiosks, automated teller machines (i.e., ATMs),data entry systems, etc.

In one specific type of touchscreen, an acoustic touchscreen, acousticor ultrasonic waves are generated and directionally propagated acrossthe touchscreen surface utilizing the phenomena of surface acousticwaves, e.g., Rayleigh waves, Love waves, or other waves. Typically eachaxis of the touch panel includes a single transmitter transducer, asingle receiver transducer, and a pair of reflective arrays. Thetransmitting transducers and the receiving transducers are coupled to acontroller, the controller generating the drive signals that are appliedto the transmitting transducers and amplifying, conditioning andresponding to the signals from the receiving transducers. The acousticwave produced by each transmitter transducer is reflected by thereflective array located near the touchscreen edge. The array reflectsthe acoustic wave, typically at a right angle along the entire length ofthe array, producing a surface acoustic wave pattern that propagatesacross the active area of the touchscreen. The propagated surfaceacoustic wave has a substantially linear wavefront with a uniformamplitude. The opposing reflective array reflects the surface propagatedacoustic wave to a receiving transducer. By monitoring the arrival timeand the amplitude of the propagated wave along each axis of thetouchscreen, the location of any wave attenuation point on thetouchscreen surface can be determined. Attenuation can be caused bytouching the screen with a finger or stylus or other media.

Typically a manufacturer of touchscreen systems produces or purchasescontrollers with a predetermined oscillation frequency that is within awell defined frequency range, the reference frequency being provided bya crystal oscillator. Then during the manufacturing process thecharacteristic frequency of each touchscreen is determined and adjusted,as necessary, to ensure that there is sufficient match between thetouchscreen and the oscillation frequency of the controller.

Let us more carefully define the characteristic frequency of atouchscreen. Acoustic touchscreens of the types of interest here havethe property of being a narrow band pass filter. The center frequency ofthe narrow band is determined by the spacing of the reflectors and bythe velocity of the acoustic waves. As a consequence, a brief burstapplied to a transmitter transducer appears, after a time delaycorresponding to an acoustic wave traveling the shortest possible pathto a receiving transducer, in the form of a long drawn-out wave train.While the frequency spectrum of the input burst is typically quite widedue to the short duration of the burst, the spectrum of the output wavetrain is ideally very narrow and sharply peaked at a specific frequency.This specific frequency is referred to as the touchscreen'scharacteristic frequency. It is desired that the touch system'soperating frequency match the touchscreen's characteristic frequency.

In principle, an ideal touchscreen has a single characteristicfrequency. In practice, manufacturing variations can result in aplurality or range of characteristic frequencies. Current practiceinvolves making a sufficient investment in the touchscreen manufacturingprocess so that there is effectively only a single characteristicfrequency of the touchscreen and that this characteristic frequencymatches that determined by the controller's reference oscillator. Inorder to achieve the desired control over the touchscreen manufacturingprocess, precise coordination of array design, careful monitoring of thesupply chains of incoming materials, and prompt electronic testing ofreflective arrays are required. In addition, when an unanticipatedchange or variation is discovered, rapid corrective action is necessary.For example, the array may need to be redesigned and a new printing maskfabricated. The degree of coordination, monitoring, and testing requiredto maintain control of the touchscreen characteristic frequency addscost to the process and limits production to facilities with a workforcewell trained in the intricacies of acoustic touchscreen manufacture.This is an important limitation of present acoustic touchscreentechnology.

In general, frequency mismatch can be categorized as being either globalor localized in nature. In cases in which the frequency mismatch isglobal, the source of mismatch affects the entire touchscreen. Forexample, if the reference oscillator of a controller drifts, oralternatively, if the glass substrate has an unexpected acousticvelocity (e.g., due to the glass substrate being fabricated by adifferent glass supplier), the frequency match between the touchscreenand the controller is compromised regardless of the location of intereston the touchscreen. In contrast, in cases in which the frequencymismatch is localized, only a specific region of the touchscreen mayexhibit mismatch with the controller.

Both global and localized frequency mismatch can be caused by a varietyof sources. Although some sources of mismatch can be overcome throughsufficient quality control, often the cost of such control can be quitehigh. For example, variations in the touchscreen glass substrate canvary the acoustic wave velocity thereby causing global frequencymismatch, controlling the glass supply chain and manufacturing processsufficiently to ensure that the acoustic wave velocity of all substratesfall within a narrow range may be economically unfeasible. Controllingthe glass supply chain and manufacturing process is even moreproblematic in those instances in which acoustic reflective arrays areprinted directly onto the faceplate of a cathode ray tube (i.e., CRT).Specific glass characteristics that are difficult to control to thedegree necessary to avoid global frequency mismatch include the chemicalcomposition and the thermal history (e.g., annealing time andtemperature, etc.).

Another source of frequency mismatch is due to undesired variationswithin the reflective array printed on the touchscreen substrate. Thesevariations may, for example, result from the array mask being distortedduring the screen printing process. Print mask distortion is especiallyproblematic if the array is to be printed directly onto a CRT faceplate.Other array printing techniques such as pad printing are also subject tothe registration errors introduced during the printing process that canlead to further frequency mismatch. Another source of frequency mismatchcan arise from improperly correcting for the spherical geometric effectsof a non-planar substrate surface.

What is needed in the art is a method and apparatus for adapting theoscillation frequency of a controller to the operating frequencyrequirements of specific touchscreens. The present invention providessuch a method and apparatus.

SUMMARY OF THE INVENTION

The present invention provides a method and apparatus for adapting thefrequency of a controller to the operating frequency requirements of aspecific touchscreen substrate, wherein the touchscreen substrateincludes reflective arrays. More specifically, the controller is adaptedsuch that it outputs a burst signal to the touchscreen's transmittingtransducers or conditions the signal from the touchscreen's receivingtransducers, thereby accommodating the particular operating frequencycharacteristics of the touchscreen's substrate.

In one application of the invention, the characteristic frequency orfrequencies of a specific touchscreen is first determined. The frequencyof the controller that is intended for use with this substrate is thenadjusted to match the substrate's measured characteristic frequency orfrequencies thus allowing the two components to be paired as a matchedset. In an alternate application, a touchscreen substrate is paired witha controller prior to matching the operating frequencies of the twocomponents. After pairing, the system is initialized during which timethe touchscreen substrate's frequency characteristics are determined.The frequency characteristics of the controller are then adjusted tomatch those of the substrate. If desired, the system can periodicallyretest the frequency characteristics of the substrate and readjust thecontroller's output as deemed necessary.

In one embodiment of the invention that is primarily intended tocompensate for global frequency mismatch errors, the adaptive controllerof the invention uses analog signal processing and a crystal referenceoscillator. A digital multiplier is used to modify the output of thereference oscillator to generate the desired frequency of the tone burstsent to the transmitting transducers and/or to vary the frequency usedby the receive circuit to produce the base-band signal. The burst lengthis determined by a burst circuit. The desired operating frequency isdetermined by a mixer containing circuit that compares the output of thedigital multiplier to the suitably conditioned output signal of thereceiving transducer. The output from the mixer containing circuit isused to determine the desired operating frequency.

In another embodiment of the invention that is intended to compensatefor both global and local frequency variations, the adaptive controllerof the invention uses digital signal processing and a crystal referenceoscillator. In this embodiment a digital signal processor receives thedigitized, filtered outputs from a pair of mixers. The inputs to themixers are a pair of reference signals, one of which has been phaseshifted by 90 degrees, and suitably filtered and amplified receivertransducer RF signals. This embodiment is an example of the use of aphase-sensitive controller in which the complete mathematical content,e.g., phase and amplitude, of the received signal is digitized. Withcomplete digitized information available for processing by digitalsignal processor algorithms, software tunable frequency filters can beapplied to the received signal. The digital signal processor, based oncorrection values contained in memory, applies a frequency filter with aspecific center frequency which preferably varies according to the delaytime since the last burst was transmitted. Thus the system can adapt tovariations caused by localized variations in the acoustic wavereflective array.

In yet another embodiment of the invention, a non-crystal localoscillator is used to provide the reference signal in the adaptivecontroller. The use of such an oscillator allows the controller to beminiaturized to a sufficient extent to allow it to be mounted directlyto a touchscreen substrate. A feedback loop is used to compensate forthe drift of the oscillator. In this embodiment the conditioned RFsignal from the touchscreen receiver transducers is mixed with theoutput from the local oscillator. The IF output from the mixer is passedto a discriminator circuit that generates a voltage, the sign of whichdepends on whether the frequency is higher or lower than desired and theamplitude of which depends on the degree of deviation from the desiredfrequency. The output from the discriminator is used to adjust thefrequency of the local oscillator such that it tracks the frequency ofthe touchscreen. To achieve the desired burst frequency, the stabilizedoutput from the local oscillator is mixed with the output from an IFoscillator.

In yet another embodiment of the invention, the burst is sufficientlybroadband so that it is sufficient to adjust only the center frequencyof the circuitry processing the receive circuit by means of a voltagecontrolled, variable frequency bandpass filter.

A further understanding of the nature and advantages of the presentinvention may be realized by reference to the remaining portions of thespecification and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of an acoustic touchscreen according to theprior art;

FIG. 2 is a graph illustrating a waveform, i.e., signal amplitude vs.time, as received by a surface acoustic wave transducer for one axis ofa touchscreen according to the prior art;

FIG. 3 is a graph of the waveform shown in FIG. 2 in which the waveformhas been perturbed by a touch on the touchscreen;

FIG. 4 is a graph of a perturbed waveform traveling across the surfaceof the touchscreen in an orthogonal direction to the waveformillustrated in FIGS. 2 and 3;

FIG. 5 is a flowchart illustrating one method of using the presentinvention;

FIG. 6 is a flowchart illustrating an alternate method of using thepresent invention;

FIG. 7 schematically illustrates an adaptive controller according to thepresent invention to correct for global variations;

FIG. 8 is a flow chart illustrating the technique used to tune thefrequency of the digital multiplier shown in FIG. 7;

FIG. 9 schematically illustrates the quadrature-sum detector;

FIG. 10 is a flowchart illustrating the methodology associated with analternate embodiment of the invention shown in FIG. 11;

FIG. 11 schematically illustrates an adaptive controller according tothe present invention to correct for both global and local variations;

FIG. 12 schematically illustrates a digital burst processor for use withthe adaptive controllers shown in FIGS. 11 and 15;

FIG. 13 schematically illustrates a controller that can be mounteddirectly to a touchscreen substrate;

FIG. 14 is a flowchart illustrating the methodology associated with analternate embodiment of the invention shown in FIG. 15;

FIG. 15 schematically illustrates an adaptive controller according tothe present invention to actively correct for both global and localvariations;

FIG. 16 schematically illustrates an adaptive controller according tothe present invention in which only the receiver center frequency isadjusted; and

FIG. 17 is an illustration of an acoustic touchscreen utilizing anadditional set of transducers for touchscreen characterization.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

FIG. 1 is an illustration of a touchscreen 100 utilizing surfaceacoustic waves according to the prior art. This type of touchscreen issuitable for use with a cathode ray tube (i.e., CRT) display, a liquidcrystal display (i.e., LCD), or other display type. A common type ofacoustic touchscreen employs Rayleigh waves, a term which, as usedherein, subsumes quasi-Rayleigh waves. Illustrative disclosures relatingto Rayleigh wave touchscreens include Adler, U.S. Pat. Nos. 4,642,423;4,645,870; 4,700,176; 4,746,914; 4,791,416; and Re 33,151; Adler et al.,U.S. Pat. Nos. 4,825,212; 4,859,996; and 4,880,665; Brenner et al., U.S.Pat. No. 4,644,100; Davis-Cannon et al., U.S. Pat. No. 5,739,479; andKent, U.S. Pat. Nos. 5,708,461 and 5,854,450. Acoustic touchscreensemploying other types of acoustic waves such as Lamb or shear waves, orcombinations of different types of acoustic waves (includingcombinations involving Rayleigh waves) are also known, illustrativedisclosures including Kent, U.S. Pat. Nos. 5,591,945 and 5,854,450;Knowles, U.S. Pat. Nos. 5,072,427; 5,162,618; 5,177,327; 5,243,148;5,329,070; and 5,573,077; and Knowles et al., U.S. Pat. No. 5,260,521.The documents cited in this paragraph are incorporated herein byreference for all purposes. Surface acoustic wave touchscreens will bediscussed briefly herein, thus allowing a fuller understanding of thepresent invention.

Touchscreen 100 includes a substrate 101 suitable for propagatingsurface acoustic waves, e.g., Rayleigh waves, Love waves, and otherwaves sensitive to a touch on the surface. To determine touchcoordinates along an x-axis 103, a system is used that includes atransmitter transducer 105, a receiver transducer 107, and a pair ofassociated reflective arrays 109 and 111, respectively. A similar systemis used to determine coordinates along a y-axis 113 which includes atransmitter transducer 115, a receiver transducer 117, and associatedreflective arrays 119 and 121. Transmitter transducers 105 and 115 arecoupled to a controller 123, typically under the control of a processor125. Receiver transducers 107 and 117 are also coupled to controller 123which includes a signal processing system 127. Although a signal may besimultaneously applied to transducers 105 and 115, preferably thesignals to the transducers are sequential, thus reducing interferenceand cross-talk between the two coordinate sensing channels. Thesequential sensing approach also reduces circuit complexity as many ofthe necessary circuits can be alternately used in the two sensingchannels thereby eliminating the need for unnecessary circuitduplication. In order to further reduce circuit complexity, typicallythe prior art sends the same frequency burst to both transmittertransducers 105 and 115.

One of the sensing channels will now be described in further detail. Thedescription of this channel is equally applicable to the second sensingchannel. In order to determine a touch coordinate along x-axis 103 forsubstrate 101, transmitter transducer 105 sends a burst acoustic wave(e.g., an approximately 5 microsecond burst) along a path 129. Due tothe relatively wide bandwidth associated with this burst acoustic wave,the frequency is not very well defined. Reflective array 109 includes aplurality of reflective elements 131 that are disposed along path 129,each element 131 oriented at an approximately 45 degree angle to path129. Elements 131 are designed to extract a plurality of wave components133 from the acoustic wave traveling along path 129, transmittingcomponents 133 along the surface of substrate 101, preferably in adirection parallel to y-axis 113. The pattern design for array 109 issuch that the individual components reflected by individual reflectors131 coherently add together, thus creating a substantially linearwavefront with uniform amplitude. Wave components 133 are recombined bya plurality of reflective elements 135 within array 111, elements 135directing the wave components along a path 137 to receiver transducer107. Individual array elements 135 are disposed along path 137 andoriented at an approximately 45 degree angle to the path. Due to thetime delay imposed on the sound wave emitted by transducer 105 by thespeed of sound associated with substrate 101, rather than a short burst,receiver transducer 107 receives a relatively long duration signal(e.g., approximately 150 microseconds duration).

Receiver transducer 107 converts the waveform information received alongpath 137 into an electrical signal. This electrical signal is analyzed,for example by performing an arrival time analysis of the received wave.FIG. 2 is a graph illustrating a typical time analysis of such a wave.As shown, the amplitude, i.e., the envelop of RF signal, of the receivedwave is plotted against time. At a time t₁, a signal is provided bysource 123 to transducer 105. Time t₂ is the beginning of the wavereceived by transducer 107. The time delay between t₁ and t₂ is due tothe time delay between the wave launched by transducer 105 reaching afirst element 139 of array 109, traveling across the surface of panel101, and being reflected by a first element 141 of array 111. At timet₃, the last of the wave reaches transducer 107. Due to the spacing anddesign of the array elements, the amplitude of the curve between timest₂ and t₃ is relatively constant, assuming that the wave is unperturbed.

FIG. 3 is a graph of a second waveform received by transducer 107. Asshown, the amplitude of the waveform has a dip 301 at a time t₁. Dip 301is due to acoustic wave attenuation at a location 143 on substrate 101.By analyzing the time delay between t₁ and t_(t), signal processor 127in conjunction with processor 125 is able to calculate the x-coordinateof touch 143. Similarly, processors 125 and 127 in conjunction withsource 123, transducers 115 and 117, and reflective arrays 119 and 121,are able to calculate the y-coordinate of touch 143. FIG. 4 is a graphof a waveform received by transducer 117 showing an attenuation dip 401due to touch 143.

FIGS. 5-6 are flowcharts illustrating the basic methodologies associatedwith the adaptive controller of the present invention. The method shownin FIG. 5 is best suited for use during the touchscreen systemmanufacturing process although it can also be employed at the user'slocation. In step 501 the characteristic frequency or frequencies of aspecific touchscreen is determined utilizing any of a variety of wellknown testing techniques. For example, if the touchscreen is beingtested at the manufacturer's site, it can be placed within a testing jigand a sound wave can be launched across the substrate's surface. Oncethe characteristic frequency or frequencies of the touchscreen is known,the frequency of the controller that is intended for use with thistouchscreen is then adjusted to match the measured touchscreen frequency(step 503). Typically the frequency of the controller is adjusted untilthe desired frequency is obtained. Alternately, the controller caninclude a look-up table into which controller settings have beenrecorded along with the resultant output frequency. Preferably thelook-up table is specific to a given controller, i.e., each controllerhas a look-up table that takes into account variations within theindividual controllers. Once the operating frequency of the touchscreenis determined, the look-up table of the controller paired with the giventouchscreen is used to make the appropriate controller settings.Regardless of which approach is used to adjust the controller, once ithas been adapted to match the substrate, the touchscreen system can beassembled (step 505).

In the method illustrated in FIG. 6, a controller is paired with atouchscreen prior to making any attempt to match the frequencies of thetwo (step 601). The touchscreen system installation is then completed(step 603) and system initialization begins (step 605). During systeminitialization, the touchscreen is tested to determine itscharacteristic frequency or frequencies (step 607). This testing steppreferably utilizes the normal touchscreen transmitter/receivertransducers (e.g., 105/107 and/or 115/117) operating in a single bursttest mode. Alternately, a dedicated pair of transducers can be used.Once the characteristic frequency or frequencies of the substrate hasbeen determined, the frequency of the controller is adjusted (step 609),making the system ready for normal operation (step 611).

In a variation of the method illustrated in FIG. 6 and described above,the system is designed to periodically adjust the controller duringtouch operation, thereby ensuring optimum frequency matching. Incontrast to the previously described system, however, a periodic testsequence is performed in which the touchscreen is retested and thecontroller readjusted (step 613). Readjustrment of the controller can beset to occur every time the system enters a power-up sequence or after apredetermined period of time has lapsed. Periodic controller adjustmentis typically desired when either the touchscreen substrate or thecontroller is prone to temperature related fluctuations. For example, ifa polymer substrate is used, the acoustic wave velocity of the substrateis likely to change with ambient temperature changes. Similarly, if thecontroller does not use a crystal oscillator, it may have a frequencyreference subject to drift, thus requiring active controller adaptation.

A readily apparent benefit of the adaptive controller described aboveoccurs whenever a touchscreen system fails at the user's location due toeither the failure of the touchscreen substrate, for example due tovandalism, or the failure of the controller. Due to the frequencyadaptive qualities of the controller, a new touchscreen or a newcontroller can easily be installed on-site, a preferable solution toeither sending the entire touchscreen system back to the manufacturerfor repair or sending a matched touchscreen/controller to the user'slocation for on-site replacement. For example, if the touchscreen of anexisting touchscreen system requires replacement, the old controller canperform a new initialization test in which the characteristic frequencyor frequencies of the new touchscreen is determined and the frequency ofthe controller is reset to match the new characteristic frequency orfrequencies. Alternately, the identification code of the new touchscreencan be used to set the controller's frequency using the look-up tabledescribed above. Similarly if a new controller is required at the user'ssite, it can either be matched to the existing touchscreen throughinitialization testing or it can be set using the look-up table approachand the old touchscreen's identification code. In the latter approachthe controller can either be set at the manufacturer's location or theuser's location.

There are a number of embodiments of the present invention, eachoffering the ability to adapt the frequency of the controller to therequirements of the touchscreen. These embodiments differ in the type offrequency mismatch that the adaptive controller corrects. The embodimentillustrated in FIG. 7 is intended for use in systems that suffer from‘global’ frequency mismatch errors, i.e., errors that uniformly affectthe degree of frequency compatibility between a controller and atouchscreen. For example, the acoustic wave velocity of a piece of glasswill typically vary depending upon its exact composition. Thus as thecomposition varies between glass batches or between glass vendors, andassuming other frequency affecting factors are sufficiently controlled,the error introduced by the compositional variation will uniformlyaffect the characteristic frequency of the entire touchscreen due tobatch to batch variations in acoustic wave velocity. In a specificexample, during the touchscreen manufacturing process a glass temperingstep is often required. Depending upon the time and temperaturecharacteristics of the glass tempering step, the characteristicfrequency may vary between individual touchscreens.

In touchscreens suffering from non-time varying global frequencymismatch errors (e.g., glass composition variations), preferably theadaptive controller of the present invention goes through a singleadaptive frequency algorithm. In this scenario multiple or continuingfrequency adaptations are not required as the mismatch between thecontroller and the touchscreen does not vary with time. Rather theintent of this embodiment is to allow a randomly selected touchscreenand a randomly selected controller (i.e., a non-pairedtouchscreen/controller set) to be successfully paired during finalsystem assembly or during system repair. Thus this adaptive frequencyalgorithm is preferably executed during the initial power-up sequence ofthe paired touchscreen/controller.

The embodiment of the adaptive controller illustrated in FIG. 7 usesanalog signal processing. It is understood, however, that digital signalprocessing could also be used in this embodiment. Within controller 700is a crystal oscillator 701 oscillating at a frequency close to thedesired frequency. The output from this reference oscillator 701 is fedinto a digital multiplier 703 (also referred to as a digital divider) aswell as a microprocessor 705 within controller 700. Digital multiplier703 mathematically modifies the output from the crystal oscillator(e.g., by multiplying the crystal oscillator frequency by a rationalnumber A/B) to generate the desired frequency based upon the commandssent to it from microprocessor 705. Thus digital multiplier 703 inconjunction with crystal oscillator 701 forms a master oscillator 704for the analog system associated with the touchscreen.

The output from digital multiplier 703 is used to generate the toneburst that is output along line 707 to the transmitter transducers(e.g., transducers 105 and 115 of FIG. 1) of the touchscreen. The toneburst is at the frequency output by multiplier 703 with a burst lengthdetermined by a burst circuit 709 coupled to microprocessor 705. Priorto the tone burst being communicated to a transmitter transducer, it istypically conditioned and amplified by a burst amplifier 711.

In order to determine the desired operating frequency, the output from areceiver transducer (e.g., 107 and 117 of FIG. 1) is sent along a line713 to a mixer containing circuit 715. Preferably the transducer outputfirst passes through a bandpass filter 717 and a RF amplifier 719.Bandpass filter 717, typically a fixed broadband filter, is primarilyused as a noise suppression circuit, conditioning the RF input. RFamplifier 719 amplifies the signal to the desired levels. Mixercontaining circuit 715 compares the frequency component of theconditioned, amplified signal from the receiver transducer to the outputsignal from digital multiplier 703, outputting a relatively slowlyvarying, substantially DC base-band signal. The output from mixercontaining circuit 715 is digitized by an A-D converter 723 and fed intomicroprocessor 705. Optionally, low pass filter 721 provides additionalconditioning of the mixer containing circuit output prior to beingdigitized, however it is typically the mixer containing circuit thatprovides the limiting narrow band filtering.

As previously noted, item 703 is preferably an A/B digital multipler. Itshould be understood, however, that in general terms item 703 is simplya frequency modifying circuit and therefore can be comprised of anydigital, analog, or mixed digital/analog electronic circuit thatmodifies the crystal oscillator frequency in response to control signalsfrom microprocessor 705.

Depending upon the application, it may be sufficient to only adapt theburst center frequency or the receive center frequency. In those casesin which it is sufficient to only adjust the burst center frequency,circuit 715 does not require input from the digital multipler 703.Accordingly it can be replaced with a more standard detector elementsuch as those commonly found in present controllers. In those cases inwhich it is sufficient to only adjust the receive center frequency, thecoupling between digital multiplier 703 and burst circuit 709 is nolonger required.

FIG. 8 is a flow chart illustrating a technique used to tune thefrequency of digital multiplier 703 to match the frequency of thetouchscreen to which it is coupled. As previously noted, preferably thisembodiment only adapts the frequency of the controller to thetouchscreen upon power-up (step 801). Alternately, the system can bedesigned to perform controller frequency adaptation periodically or onlyduring the first power-up cycle.

After power-up step 801, microprocessor 705 sweeps the output of digitalmultiplier 703 through a predetermined frequency range (step 803).Preferably the controller performs a course tuning operation first,followed by a fine tuning operation, although it is possible to combinethese two operations into a single scan sequence. Therefore during step803, the predetermined frequency range is scanned using relatively largefrequency steps. The output of A-D converter 723 for each frequency stepis summed (step 805) and the maximum signal amplitude is selected (step806), indicating the closest match between the output of the masteroscillator and the touchscreen. This scanning/optimization process isthen repeated (steps 807-809), scanning the output frequency of themaster oscillator around the previously selected frequency using smallerfrequency steps. The frequency determined in step 809 to be closest tothe touchscreen natural frequency is then entered into memory (step811), thus ensuring that the output of the master oscillator circuit ismaintained at the desired frequency.

Although a two step frequency scanning approach is shown in FIG. 8, itwill be understood by those of skill in the art that there are numerousother techniques for determining the desired output frequency. Forexample, the present invention can also utilize a dithering orsuccessive approximation approach.

The basic algorithm of FIG. 8 does not require the use of an A-Dconverter sum. In general, steps 805 and 808 represent the collection ofany measurable quantity that is sensitive to the degree of frequencymismatch while steps 806 and 809 represent the selection of themeasurable quantity corresponding to an acceptably small frequencymismatch. For example, for a given time interval microprocessor 705 cancount the number of RF cycles in both the received signal and in theoutput of digital multiplier 703. The difference between the number ofRF cycles provides a measure of frequency mismatch. Other circuits andtechniques that accomplish the same purpose are well known by those ofskill in the art.

FIG. 9 schematically illustrates a quadrature-sum detector as anillustrative example of a mixer containing circuit 715. Conditioned RFinput signal 901 is mixed with the oscillator output from digitalmultiplier 703 in a mixer 903. Mixer 903 outputs the sum frequency andthe difference frequency of the two input frequencies. The sumfrequency, at approximately 10 MHz, is filtered out using a low passfilter 905. The remaining frequency is close to zero, i.e., base-band.Although the single mixer circuit described above can be used to providea base-band signal, the output is dependent upon the relative phases ofthe oscillator output and the RF input signals. In order to achieverelative phase independence, i.e., to avoid beat patterns in thewaveforms digitized by A-D converter 723, the quadrature-sum detectorhas two channels as illustrated in FIG. 9. As shown, a second mixer 907is used in which the frequency input from the oscillator is phaseshifted by 90 degrees. The output of second mixer 907, after passingthrough another low pass filter 905 is summed with the output from thefirst channel in a quadrature summing circuit 909. The output of circuit909 is a base-band signal 911 that is free of beat patterns and isindependent of the exact phase of the received signal. Effectively, thequadrature-sum detector of FIG. 9 provides a narrow bandpass filterwhose center frequency is adjustable and controlled by the frequency ofthe output of the digital multiplier 703.

Mixers 903 and 907 shown in FIG. 9 are essential components of thequadrature-sum detector as well as of other possible mixer containingcircuits. Mixer 903, for example, combines the signal originating online 713 and the output of source 704 to obtain a desired output whichis a function of both input signals and of the difference between theirfrequencies. In some cases the full quadrature-sum detector may not berequired. For example, if adjustment of the burst frequency is all thatis required, the output quantity of interest is the beat of differencebetween the signals from line 713 and from source 704. Such a differencefrequency signal can easily be produced by a diode mixer. Other mixingdevices are, of course, known in the art and may be used in modifiedforms in this invention.

In an alternate embodiment of the invention illustrated in FIGS. 10-12,the controller is programmable in a manner that allows it to adapt toboth global variations, i.e., frequency variations that uniformly affectthe characteristic frequency of the entire touchscreen, and localizedvariations, i.e., frequency variations within a localized region of thetouchscreen. For illustrative purposes only, this embodiment utilizesdigital signal processing. It should be understood, however, that thisembodiment could be implemented using analog signal processing as well.

FIG. 10 is a flowchart illustrating the methodology associated with thisembodiment of the invention. After fabrication of a touchscreensubstrate is complete, including any required array deposition and glasstempering steps, the characteristic frequencies of the touchscreen,including effects of any localized array distortions, are measured (step1001). Preferably these measurements take place within the manufacturingplant using production floor test equipment. Based on thesemeasurements, a series of frequency correction values are calculated(step 1003), typically as a function of delay time for both the x- andy-coordinates. This set of correction values, specific to an individualtouchscreen, are then loaded into the memory of adaptive controller 1100(step 1005) which is paired with this particular touchscreen (step1007). It is understood that steps 1005 and 1007 can be reversed insequence and that touchscreen substrate frequency variation measuringstep 1001 can be combined with correction value calculation step 1003.

In a slight variation of the method illustrated in FIG. 10, eachtouchscreen substrate is provided with an identification code. A tableof identification codes and the associated correction values particularto each identification code are then archived, preferably by themanufacturer, seller, or both. Thus if there is ever a need to replacethe controller, for example due to breakage, the user need only supplythe identification code in order to obtain a new controller which hasbeen preloaded with the necessary correction values.

In the embodiment of the invention illustrated in FIGS. 11 and 12,adaptive controller 1100 uses an oscillator 1101 as a reference.Preferably a stable crystal oscillator is used as the frequency source.The output from oscillator 1101 is sent to a frequency divider/phaseshifter 1103 which divides the frequency from a frequency ofapproximately 22 MHz to the desired frequency of approximately 5.53 MHzand phase shifts a portion of the output by 90 degrees. Unshiftedoscillator frequency 1105 and phase shifted oscillator frequency 1107are then mixed in mixers 1109 and 1111 with suitably filtered andamplified receiver transducer RF signals. As in controller 700, the RFsignal from the touchscreen's receiver transducers are filtered with aband pass filter 1113, typically a fixed broadband filter, to removevarious noise components and then amplified by amplifier 1115 in orderto achieve the desired signal levels.

The output of mixers 1109 and 1111 represent the x- and y-signalamplitudes in the complex plane. Thus by using a pair of mixers and apair of reference signals, one of which has been phase shifted by 90degrees, the phase as well as the phase independent magnitude of thecomplex number can be determined. The output of mixers 1109 and 1111 arepassed through a pair of low pass filters 1117 and 1119, respectively,and then digitized with A-D converters 1121 and 1123, respectively.These signals are then sent into a digital signal processor (i.e., DSP)1125.

DSP 1125 acts as a frequency filter in which both the center frequencyand the bandwidth are mathematically controllable. Methods ofmathematically controlling DSP 1125 to achieve a controllable bandwidthand center frequency are well known by those of skill in the art andwill therefore not be discussed in detail herein. Coupled to DSP 1125 isa memory 1127. Memory 1127 contains the correction values that areobtained by measuring the frequency characteristics of a particulartouchscreen (i.e., the touchscreen which is to be paired with controller1100). Based on the correction values contained in memory 1127, DSP 1125responds to a specific center frequency. Preferably DSP 1125 responds toa center frequency which varies according to the delay signal, thustaking into account variations caused by localized variations in theacoustic wave reflective array.

Item 1125 of FIG. 11 is a digital signal processor (i.e., DSP) in thegeneral meaning of the word. It represents mathematical or digitalprocessing of the digitized signals from A-D converters 1121 and 1123.DSP 1125 can be implemented in many ways. For example, DSP 1125 may becode executed by microprocessor 1131. Alternately, DSP 1125 may bedigital circuitry custom designed for acoustic touchscreen controllers.Furthermore, digital signal processing may take place in a packagedsilicon chip of the type often referred to as a “DSP chip” by electronicengineers although it is understood that it is not so limited.

To provide a transmit transducer burst, the output from crystaloscillator 1101 is fed into a digital burst circuit 1129. Burst circuit1129 manipulates this signal according to the instructions received frommicroprocessor 1131 which, in turn, receives instructions regarding thedesired center frequency from permanent memory 1127. The output ofdigital burst circuit 1129 is amplified, if needed, by a burst amplifier1133 prior to being sent to a transmitter transducer along line 1135.

FIG. 12 is a schematic illustration of an example of digital burstcircuit 1129. Within burst circuit 1129 is a bit register 1201 (e.g.,64×8 bit register) coupled to microprocessor 1131. Microprocessor 1131loads the desired bit pattern (i.e., the digital pattern generated bymicroprocessor 1131 in response to the output from permanent memory1127) into register 1201, the bit pattern determining the burst centerfrequency. For each burst, the bit pattern loaded into register 1201 islatched into a shift register 1203 which, to create a burst, is clockedout. It should be understood that different bit patterns can be used todetermine the burst center frequency for the x- and y-coordinates of thetouchscreen, thus taking into account variations between the two axes.It should be noted that the bit pattern can either be calculated bymicroprocessor 1131 in response to frequency correction data from memory1127 or be stored directly in memory 1127.

In another example that can utilize this embodiment of the adaptivecontroller, the touchscreen uses grating transducers. In a gratingtransducer, the piezoelectric element is applied to the back surface ofthe substrate and a grating applied to the front surface of thesubstrate. The grating is used to coherently diffract the pressure wavecreated by the piezoelectric element, thus generating an acoustic wavetraveling along the surface of the surface. Such grating transducers arefound to be most efficient when the operating frequency corresponds to aglass thickness resonance. As the glass thickness resonance frequency ofthe substrate is dependent upon the thickness of the substrate,preferably the glass thickness is first measured, then the optimaloperating frequency is calculated and an appropriate reflective arrayand grating design for the optimal operating frequency is applied. Theadaptive controller of the present invention, e.g., controller 1100, isthen used to match the frequency of the controller to the frequencycharacteristics of the touchscreen. Unlike some applications of thisembodiment, however, this example requires that the adaptive controllerhave the ability to vary the burst frequency as much as 10-20 percentfrom the frequency of the reference oscillator. Any receive bandpassfilter, e.g., filter 1113, needs either to be tunable or to besufficiently broadband to cover the fall range of variation oftouchscreen frequency characteristics.

FIG. 13 schematically illustrates an alternate embodiment of an adaptivecontroller that can be mounted directly to a touchscreen substrate thusoffering both size and cost benefits. In this embodiment the crystaloscillator is replaced with a local oscillator 1301, thus providing thedesired size. Local oscillator 1301 may, for example, be constructedentirely from circuit components on a silicon chip. Given the drift oflocal oscillator 1301 relative to a crystal oscillator, a feedback loopis required to provide the required frequency stability. As a result ofthe feedback loop, controller 1300 actively, i.e., repeatedly, adaptsthe oscillator frequency to the desired frequency.

As in the previous embodiments, the RF signal from the touchscreenreceiver transducers is first conditioned by passing it through abandpass filter 1303 and an amplifier 1305. The conditioned RF signal ismixed with the output from local oscillator 1301 in a mixer 1307.Oscillator 1301 is a variable frequency oscillator in which thefrequency is controlled by, for example, an input voltage. With anappropriate buffering circuit between capacitor 1308 and oscillator1301, oscillator 1301 can provide other types of electronic input suchas current. In this embodiment the local or reference oscillator isoperating at a frequency greater than the touchscreen frequency. Forexample, for a nominal touchscreen frequency of 5.5 MHz, oscillator 1301may operate at a frequency of approximately 6 MHz. The output from mixer1307 will then be at an IF frequency of approximately 500 kHz.

The IF output from mixer 1307 passes through a bandpass filter 1309prior to entering a discriminator 1311. Discriminator 1311 generates avoltage, the sign of which depends on whether the frequency is higher orlower than the center frequency of discriminator 1311 and the amplitudeof which depends on the degree of deviation from the discriminator'scenter frequency. The output from discriminator 1311 is then used toadjust the frequency of local oscillator 1301, for example using avaractor diode, to reduce the discriminator output voltage to near zero.A switch 1313 coupled to a control processor 1314 is part of a sampleand hold circuit that allows local oscillator 1301 to be held at apreviously determined frequency between burst/receive cycles. Switch1313 is closed during receive cycles.

During system power-up, local oscillator 1301 may be off of the desiredfrequency by a considerable margin, thus preventing the feedback loopfrom effectively stabilizing the oscillator. As such, controller 1300preferably includes a ramping feature that gradually adjusts thefrequency of local oscillator 1301 until the feedback loop can takeover. In one mode of operation, during power-up switch 1313 is open anda second switch 1315 is closed. A digital to analog converter (i.e., aDAC) 1317 under the control of microprocessor 1314 adjusts the frequencyof oscillator 1301, increasing (or decreasing) that frequency whileobserving the output of mixer 1307 with a detector 1319. Detector 1319is coupled to microprocessor 1314 via an A-D converter 1321. When theoutput of detector 1319 exceeds a predetermined threshold, therebyindicating that local oscillator 1301 is close to the desired frequency,microprocessor 1314 opens switch 1315 and closes switch 1313, allowingthe feedback loop to fine tune the local oscillator frequency.Alternately, both switch 1313 and switch 1315 can be closed duringpower-up. In this mode once the oscillator frequency is within thebandwidth of bandpass filter 1303, microprocessor 1314 opens switch 1315thus allowing the feedback loop to fine tune the frequency from thispoint forward.

In contrast to the previously described embodiments, the frequency oflocal oscillator 1301 is not adjusted to the desired burst frequency.Rather, the frequency of local oscillator 1301 tracks the frequency ofthe touchscreen so as to maintain a fixed difference (in this example,500 kHz) between the two frequencies. Therefore in order to achieve thedesired burst frequency, the stabilized output from local oscillator1301 is mixed in a second mixer 1323 with the output from an IFoscillator 1325. IF oscillator 1325 operates at the same frequency as IFbandpass filter 1309 (i.e., approximately 500 kHz in this example). Theoutput from mixer 1323 is at the desired burst frequency (i.e.,approximately 5.5 MHz in this example). A bandpass filter (not shown)may be inserted between mixer 1323 and burst circuit 1327 to pass onlythe desired sum or difference frequency from mixer 1323. As in theprevious embodiments, the length of the tone burst at this frequency iscontrolled by a burst circuit 1327 coupled to microprocessor 1314. Thetone burst is typically amplified to the desired amplitude by a burstamplifier 1329 prior to being output along a line 1331 to one of thetouchscreen's transmitter transducers.

The circuit in FIG. 13 is an example of a circuit that shifts thereceive signal from the RF frequency to a lower frequency that is notnecessarily bass band, e.g., 500 kHz. This is a general techniqueavailable to the designer of adaptive frequency controllers. The choiceof this lower frequency can be anywhere between the RF frequency andbassband. Its optimal value depends on the details of the specificcircuit, noise sources, etc.

FIGS. 14-15 illustrate another embodiment of the invention ideallysuited for touchscreens that experience variations in the substrateacoustic wave velocity during use. For example as previously noted, theacoustic wave velocity characteristics of a polymer substrate may betemperature dependent. Therefore during use a polymer substrate basedtouchscreen may display both global variations (e.g., due to a change inthe overall room temperature) or local variations (e.g., due todifferent portions of the screen being at different temperatures). Theembodiment illustrated in FIGS. 14 and 15 is designed to accommodatesuch variations.

FIG. 14 is a flowchart illustrating the methodology of an embodiment inwhich a controller 1500 is coupled to a touchscreen requiring activeadaptation. In this embodiment the first step is to determine whether ornot a touch is sensed by the touchscreen (step 1401). If no touch issensed, controller 1500 undergoes a testing sequence in which thefrequency characteristics of the touchscreen are determined. Preferablythe first step in this sequence is to determine how much time has passedsince the last testing sequence (step 1403). If a preset time period hasnot been exceeded (step 1405), the system loops back to the startingpoint. If the preset time period has been exceeded, then the systemmeasures the substrate frequency characteristics for the x- andy-coordinates of the substrate (step 1407) and determines a set ofcorrection values (step 1409). These correction values are loaded intothe memory of controller 1500 (step 1411) and the system loops back tothe starting point (step 1413). Then, once a touch is sensed (step1415), the system determines the touch coordinates (step 1417) and sendsthese coordinates to the operating system (step 1419).

Active adaptive controller 1500 is shown in FIG. 15. This controller isbasically the same as controller 1100 except for a couple of minoralterations. For example, permanent memory 1127 is replaced by atemporary memory 1501. As in controller 1100, memory 1501 stores thefrequency correction values required to correct for the characteristicfrequency variations of the touchscreen. A temporary memory is requiredin this embodiment as controller 1500 periodically updates thecorrection values as described above. Additionally, as the memory mustbe periodically updated, it is bidirectionally linked to microprocessor1131. Thus during the characteristic testing sequence, microprocessor1131 uses the output of DSP 1125 to determine the desired frequencycorrection values, storing them in memory 1501.

As in the embodiment illustrated in FIG. 11, digital burst processor1129 outputs a burst of the desired burst frequency. Additionally thepower spectrum of the output burst is tailored according to thecorrection values stored in temporary memory 1501. A variety oftechniques can be used to adjust the burst power spectrum including timemodulating the phase of individual RF pulses (e.g., pulse phasing basedon sin(x)/x curve), amplitude modulating a burst train (e.g., trapezoidenvelop or stacking of digital pulse trains of different lengths), orusing non-integral burst lengths in units of RF cycles.

In the embodiment illustrated in FIG. 16, only the center frequency withwhich the received signal is processed is adjusted, i.e., the frequencyof the burst is not adjusted. This embodiment is applicable in cases inwhich there is no need to adjust the burst frequency, such as if theburst is very short, e.g., less than 10 RF cycles in duration, and istherefore sufficiently broadband to cover anticipated variations in thetouchscreen characteristic frequencies.

As shown in FIG. 16, a microprocessor 1601 accepts the nominal RFoperating frequency and triggers a burst circuit 1603 which, in turn,excites a transmit transducer (not shown). As in the previousembodiments, a burst amplifier 1605 may be used to condition the outputof burst circuit 1603. The narrowest bandpass filter in the receivercircuit chain is a variable bandpass filter 1607. The center frequencyof variable bandpass filter 1607 is controlled by a voltage provided bya D-A converter 1609 which, in turn, is controlled by microprocessor1601. Appropriate circuit designs for a variable bandpass filter such asfilter 1607 are well known by those of skill in the art and willtherefore not be further described. The signal from the receivetransducer (not shown) may be passed through a relatively broad bandpassfilter 1611 and amplified by an amplifier 1613 prior to passing throughvariable bandpass filter 1607, filter 1607 defining the centerfrequency. The signal is then converted from RF to baseband by adetector 1615 and digitized with an A-D converter 1617. Microprocessor1601 determines an optimal setting for D-A converter 1609, for exampleby using the procedure illustrated in FIG. 8. The optimal D-A convertersetting is then stored in a memory 1619, microprocessor 1601 using thestored value during normal touch operation. Separate D-A convertervalues may be stored for x- and y-signals.

Preferably in each of the embodiments disclosed above, the transducersused during touch sensing, e.g., transducers 105, 107, 115, and 117, arealso used to adapt the controller to the touchscreen. Thus, for example,the received signal resulting from an acoustic wave launched bytransducer 105 and received by transducer 107 could either be used as afrequency reference for the adaptive controller of the present inventionor to provide touch information in a manner which is the same as, orsimilar to, a conventional touchscreen. It should be understood,however, that the transducers that are used to determine thecharacteristic frequencies of the touchscreen in order to adapt thecontroller need not be the same as the transducers used for touchdetection and information gathering. For example, as shown in FIG. 17, apair of transducers 1701 and 1703 are used in a delay line feedbackoscillator (not shown) to determine the characteristic frequencies ofthe touchscreen, these transducers being in addition to transducers 105,107, 115, and 117 that are used during touch sensing. Alternately,separate transducers with separate reflective arrays can be provided onthe back surface of the touchscreen substrate. Preferably the input andoutput of the additional transducers are multiplexed with lines 707 and713 of controller 700, or the corresponding lines of controllers 1100,1300, or 1500. This approach provides the freedom to optimize propertiesof the frequency reference signal independently of the needs of thetouch sensing acoustic paths.

Although several embodiments of the invention have been described andillustrated above, it should be understood that other embodiments can beenvisioned that use the adaptive methodology of the present invention.Additionally, it should be understood that various aspects of theembodiments shown above can be changed without departing from theinvention. For example, the non-crystal reference oscillator andfeedback loop used in the embodiment illustrated in FIG. 13 could beused in lieu of the crystal oscillator used in the embodimentillustrated in FIGS. 11-12. Thus the present invention may be embodiedin other specific forms without departing from the spirit or essentialcharacteristics thereof. Accordingly, the disclosures and descriptionsherein are intended to be illustrative, but not limiting, of the scopeof the invention which is set forth in the following claims.

what is claimed is:
 1. A touchscreen system, comprising: a touchscreensubstrate, said substrate capable of propagating acoustic waves; atleast one transmitting transducer coupled to said substrate, saidtransmitting transducer initiating an acoustic wave of a first burstlength in response to an input signal; a reflective array patterncomprised of a plurality of acoustic wave reflectors coupled to saidsubstrate, said reflective array stretching said first burst length toform a second burst length acoustic wave; at least one receivingtransducer coupled to said substrate, said receiving transducerreceiving said acoustic wave of said second burst length; and anadaptive controller coupled to said at least one transmitting transducerand to said at least one receiving transducer, said adaptive controllercomprising: a reference oscillator outputting a frequency; a first mixercoupled to said at least one receiving transducer and to said referenceoscillator, said first mixer outputting a first mixer output signal; anIF bandpass filter coupled to said first mixer; a feedback loop circuitcoupled to said IF filter and to said reference oscillator, saidfeedback loop circuit modifying said reference oscillator frequency; asecond mixer coupled to said reference oscillator; an IF oscillatorcoupled to said second mixer; a microprocessor; and a burst circuitcoupled to said second mixer, said microprocessor, and said at least onetransmitting transducer, said burst circuit outputting said input signalto said at least one transmitting transducer.
 2. The touchscreen systemof claim 1, said feedback loop circuit further comprising adiscriminator circuit coupled to said IF bandpass filter, saiddiscriminator circuit outputting a control signal to said referenceoscillator for controlling said reference oscillator frequency.
 3. Thetouchscreen system of claim 2, said feedback loop circuit furthercomprising a sample and hold circuit.
 4. The touchscreen system of claim1, further comprising a detector coupled to said IF bandpass filter andto said microprocessor.
 5. The touchscreen system of claim 1, wherein anIF oscillator frequency is substantially equivalent to a passband centerfrequency of said IF bandpass filter.
 6. The touchscreen system of claim1, wherein said reference oscillator frequency is greater than acharacteristic frequency of said touchscreen.
 7. The touchscreen systemof claim 1, wherein at least a portion of said adaptive controller isbonded to said touchscreen substrate.
 8. The touchscreen system of claim1, wherein said reference oscillator is comprised of an oscillatorcircuit.
 9. The touchscreen system of claim 8, wherein said referenceoscillator further comprises a silicon chip, said silicon chipcontaining said oscillator circuit.
 10. A method of controlling atouchscreen system including a touchscreen having a characteristicfrequency, the method comprising the steps of: generating a firstfrequency, wherein said first frequency is adjustable; mixing an outputfrom a receiving transducer with said first frequency to produce asecond frequency; passing said second frequency through a band passfilter; comparing said filtered second frequency to said touchscreencharacteristic frequency; and adjusting said first frequency until saidfiltered second frequency is substantially equivalent to said desiredfrequency.
 11. The method of claim 10, wherein said second frequency isapproximately 500 kHz.
 12. The method of claim 10, further comprisingthe steps of: setting said first frequency to an initial frequency uponsystem initialization; ramping said first frequency from said initialfrequency through a range of frequencies; and discontinuing said rampingstep when said filtered second frequency is within a predetermineddistance from said desired frequency.
 13. The method of claim 10,further comprising the steps of: mixing said first frequency with an IFoscillator output to produce a third frequency; transmitting said thirdfrequency to a burst circuit; generating a burst signal; andtransmitting said burst signal to a transmitting transducer coupled tosaid touchscreen system.